Large steam turbines of the type used by the utility companies for producing electrical power may be advantageously operated with a steam bypass system to divert excess steam from the turbine to pass directly to the condenser under certain operating conditions. The bypass mode of operation permits the steam generator to be maintained at a high steam production rate and pressure regardless of the load demand on the turbine as excess steam is bypassed during periods of low turbine loading. As load on the turbine is increased, more steam flow can be apportioned to it and less bypassed until a point is reached at which all of the steam is devoted to the turbine and none bypassed. Once the bypass is completely shut off, coordinated boiler control maintains a desired pressure-flow characteristic and increased turbine demand for steam may, for example be satisfied by allowing the boiler pressure to increase, or slide upward, in support of the increasing load. As load on the turbine is lessened, the boiler pressure may then be allowed to decrease to some acceptable minimum level as excess steam is again bypassed around the turbine.
The principal advantages of this mode of operation are believed to be:
(1) shorter turbine startup times; PA1 (2) use of larger turbines for cycling duty for quicker responses to changes in load; PA1 (3) avoidance of boiler tripout with sudden loss of load; PA1 (4) reduction of solid particle erosion; PA1 (5) enables the boiler to be operated independently of the turbine; and PA1 (6) allows the boiler to be more stably operated with better matching of steam to turbine metal temperatures.
A general discussion of the sliding pressure, or bypass mode of operation appears in Vol. 35, Proceedings of the American Power Conference, "Bypass Stations For Better Coordination Between Steam Turbine and Steam Generator Operation", by Peter Martin and Ludwig Holly.
Contrasted with the more conventional mode of turbine operation (wherein the boiler generates only enough steam for immediate use and where there are no bypass paths), the bypass mode of turbine operation necessitates unified control of a more complex valving arrangement. The control system must provide precise coordination and control of the various valves in the steam flow paths and do so under all operating conditions while maintaining appropriate load and speed control of the turbine.
Various control systems have been developed for reheat steam turbines operating in the bypass mode. In one known scheme, pressure in the first stage of the turbine is used as an indicator signal of steam flow from which reference set points are generated for control of the high pressure and low pressure bypass valves. There are no provisions in this scheme, however, for directly coordinating operation of the bypass valves with operation of the main control valve which must be responsive to speed and load requirements, nor are there provisions for coordinated operation with other valves of the system. Furthermore, it is recognized that first stage pressure is not a valid indicator of steam flow under all prevailing conditions.
In another known control system for bypass steam turbines, a flow measuring orifice in the main steam line provides a signal indicative of total steam flow which forms the basis for a pressure reference signal for control of the high pressure and low pressure bypass valves. The principal disadvantage of this system is that the flow measurement requires an intrusion into the steam flow path which causes a pressure drop and loss in heat rate.
In U.S. patent application Ser. No. 046,865, now U.S. Pat. No. 4,253,308 assigned to the assignee of the instant invention, Eggenberger et al discloses and claims a comprehensive control system for a steam turbine and bypass system which is much improved over the prior art and in which an actual load demand (ALD) signal is generated to produce independent pressure reference functions for control of boiler and reheat pressure. The ALD signal is a measure of actual steam flow to the turbine and is obtained by taking the product of boiler pressure and an admission control valve positioning signal generated by the speed and load control loop. The ALD signal provides an accurate measure of steam flow without the necessity of having a flow sensor installed in the steam line with the attendant pressure drop and loss in heat rate. Furthermore, in contrast to indirect methods of steam flow measurement such as sensing turbine first stage pressure, the ALD signal is a valid indicator of steam flow to the turbine under all operating conditions. The disclosure of the above-mentioned U.S. Pat. No. 4,253,308 is hereby incorporated herein by reference.
Viewed strictly as a control system for a steam turbine and bypass system operating over a narrow range of boiler steam flow conditions, the above-mentioned control system of Eggenberger materially advances the art of turbine bypass control systems. However, with the bypass mode of operation being extended to ever larger turbines operating over a wider range of flow conditions and with the requirement that the bypass system be capable of handling the entire steam supply, it becomes imperative that the turbine and bypass system be controlled so that the boiler is not subjected to widely varying steam flow rates that produce large fluctuations in boiler pressure. It is particularly important that the boiler be immunized from the effects of turbine transient conditions such as a sudden turbine trip. Prior art control systems have not adequately dealt with these problems without some sacrificing in heat rate.
Additionally, and particularly with larger turbines, the steam condenser and last stages of the high pressure section of the turbine are subject to high temperature effects under certain operating conditions associated with the bypass mode of operation. One aspect of the problem of high temperatures in the last stages of the high pressure section (known as "windage loss heating"), is dealt with by a reverse steam flow system disclosed and claimed in copending application Ser. No. 105,019, now U.S. Pat. No. 4,309,873, which is of common assignee with the instant application, and whose disclosure is hereby incorporated herein by reference. To fully protect both the condenser and the last stages of the high pressure section, however, rational limitations must still be imposed on the steam flow which passes through the bypass system around the lower pressure sections of the turbine. Although such limitations are required, they should not interfer with turbine control but should guard against potential overheating in the condenser and last stages of the high pressure section of the turbine such as may occur with excessively high rates of steam flow by passing lower pressure sections of the turbine.
Accordingly, it is the general objective of the present invention to provide a control system for a reheat steam turbine and its associated bypass system in solution to the problems outlined above. More specifically, it is sought to provide a system for precise and comprehensive control of a bypass steam turbine so that boiler pressure and steam flow may remain substantially free from the effects of transient turbine operating conditions.
Another specific objective of the present invention is to provide a control system for a bypass steam turbine which turbine includes means for reverse steam flow through the high pressure turbine section to prevent windage loss heating.
A still further objective of the invention is to provide a turbine control system having means to control the steam flow bypassing lower pressure sections of the turbine so that overheating of the condenser and latter stages of the high pressure turbine section due to excessive steam flow rates is prevented.